1. Field of the Invention
This invention relates to methods for regenerating catalysts and, more particularly, to a method for regenerating an oxidation catalyst used in preparing unsaturated aldehydes or acids from the corresponding olefins or aldehydes by gas-phase oxidation.
2. Description of the Prior Art
Oxidation catalysts which are used for producing an unsaturated aldehyde or unsaturated acid from the corresponding olefin or aldehyde by gas-phase catalytic oxidation often lose their activity in use for various reasons such as abnormal reactions, change of structure during a lengthy reaction period, etc.
Such catalysts can also lose their activity if the temperature of the heat treatment before use is too high.
A phosphorus-molybdenum-alkali metal catalyst which can be used in the present invention is disclosed by the present inventors in Japanese Published Unexamined Patent Applications Nos. 41811/75, 142510/75 and 100019/75. This catalyst has excellent performance in the production of unsaturated carboxylic acids from unsaturated aldehydes, but it can lose its activity for the reasons mentioned above.
Such a deactivated catalyst is generally regenerated indirectly by separating and recombining its constituents by a chemical process, but this procedure is very uneconomical from an industrial point of view.
For directly regenerating the deactivated catalyst, various methods are known. For example, a catalyst containing phosphorus and vanadium is regenerated by treatment with halogen or halide as disclosed in Belgian Pat. No. 846,608; a catalyst containing phosphorus and molybdenum is regenerated by treatment with aqueous ammonia as described in Japanese Published Examined Patent No. 33082/72; a molybdenum-bismuth catalyst is regenerated by treatment with aqueous ammonia as described in Japanese Published Examined Patent No. 27751/68; and a molybdenum-vanadium-arsenic catalyst is regenerated by treatment with aqueous ammonia as disclosed in Japanese Published Examined Patent No. 23615/70. However, the cataysts regenerated by these methods are still insufficient in performance and therefore, a need continues to exist for a more effective regenerating method.